1. Field of the Invention
The invention relates generally to electrical control systems, and more particularly to systems and methods for controlling variable speed drives of the type used in connection with oil production equipment.
2. Related Art
Oil is typically produced by drilling a well into an oil reservoir and removing the oil from the reservoir through the well. If the pressure in the well is insufficient to force the oil out of the well, an artificial lift system such as an electric submersible pump (ESP) or an above-ground horizontal pump may be used to pump the oil out of the well. Electrical power is provided to an electrical drive system at the surface of the well and, in turn, this drive system provides the required electrical power to the pump.
Commonly, drive systems for downhole equipment such as an ESP receives power from a local source at the surface of the well and transforms the power to a different form that is suitable to drive the downhole equipment. A typical variable speed drive for an ESP has a converter section, a capacitor bank and an inverter section. The converter section converts power from a local AC power supply to DC power and supplies this power to the capacitor bank. The energy stored in the capacitor bank is then used by the inverter section to produce an AC or pulse width modulated (PWM) output voltage that has characteristics (e.g., voltage and frequency) that are specifically designed to drive the ESP at a desired speed.
The operation of these electric drive systems is occasionally interrupted. This may be the result of an interruption in the supply of power to the system, a need to perform repair or maintenance on the system, or various other reasons. When the system is restarted, the capacitor bank has typically discharged, either completely or partially, so the restart may cause a sudden inrush of current to the discharged capacitor bank. If this inrush of current is not controlled, it may damage the components of the drive system, such as fuses.
It would therefore be desirable to provide systems and methods for controlling the inrush of current to the capacitor bank of an electric drive system when the system is started or restarted.